extra data at end of packet

I've been looking closely at the data coming from lwip using tcpdump and
I have a couple of questions:

        1) On some tcp packets - usually 1 byte or 0 byte packets there is
extra data at the end.  The IP header indicates the correct length, but
there will be 5 bytes of extra data if the tcp packet size is 1 byte or
6 bytes of extra data if the tcp packet size is 0 (e.g. for an ACK). 
This extra data seems to consist of the window, checksum, and urgent
pointer field data - those fields are repeated.  It's as if lwip has to
send at least 6 bytes of data in every tcp packet.  An example is a 1
byte tcp packet: the IP header shows a total length of 41 bytes, the
internet header length is 5 32bit words, the tcp header length is 5 32
bit words, and there is 1 byte of data - however the packet itself has 5
bytes of extra data that consist of the window field, then the checksum
field, and then the first byte of the urgent pointer field.

        2) In looking at the transmission of tcp packets between a server and
lwip I see an awful lot of retransmissions done by lwip.  I see repeated
packets several times.  Is there a setting for how often or how soon
lwip performs a retransmission of a packet?